1-liner: compact creative solution for Median of Three by przemyslaw.daniel - python coding challenges

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1-liner: compact solution in Creative category for Median of Three by przemyslaw.daniel

median_three = lambda d: d[:2]+[sorted(x)[1] for x in zip(d, d[1:], d[2:])]

Dec. 25, 2018

Comments:

liw_80 on July 29, 2019, 2:34 a.m.
Beatiful

maybe on Jan. 12, 2020, 12:27 a.m.
only works, if iterable is multi-pass like lists

koborC on Jan. 5, 2021, 2 p.m.
Hello, could you teach me what multi-pass is? Or give me exaple of what classes are multi-pass and what are not.. I'm still a newbie and Cannot understand your point.

maybe on Jan. 5, 2021, 10:35 p.m.
The wording "multi-pass" is not exactly in the Python "vocabulary", but I try to explain what I mean by this in my own words. The mission looks for solutions that work for <a href="https://docs.python.org/3/glossary.html#term-iterable">iterables</a> (also discussed <a href="https://stackoverflow.com/questions/32799980/what-exactly-does-iterable-mean-in-python-why-isnt-my-object-which-implement">here…</a> ), not only sequences. An "iterable" is something you can iterate over, i.e. you can access the elements one by one (like in "for x in iterable" or "map(somefunc, _iterable)" ). These iterables come in two flavors: * iterator likes (something with an _ iter()_ method), example: generator, set, map object * <a href="https://docs.python.org/3/glossary.html#term-sequence">sequences</a> (that support indexed access like "my_seq[3]" or slices "aaa[1:]"), example: lists, tuples This solution is fine with sequences, but not with iterator likes. Formally the algorithm fails then, because there are no slices. But the "real" reason is that iterators (in Python) are only meant to be run through once (one pass) and are then exhausted (empty, nada), whereas this solution needs 3 copies of the data (pass through it multi times "multi-pass"). checkio only checks for sequences, so the author got away with it. An easy workaround is to transform d into a list (d=list(d)), but that would require a clumsy def (but still a one-liner): def median_three(d): d=list(d);return d[:2]+[sorted(x)[1] for x in zip(d, d[1:], d[2:])] Example for "iterator likes" are for instance generators: <pre class='brush: python'>>>> it = (i*i for i in range(10)) >>> type(it) <class 'generator'> >>> list(it) [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] >>> list(it) [] <-- note: exhausted >>> it[1:] <-- note: no slices Traceback (most recent call last): File "<console>", line 1, in <module> TypeError: 'generator' object is not subscriptable >>> it1 = it = (i*i for i in range(10)) >>> list(zip(it,it1)) [(0, 1), (4, 9), (16, 25), (36, 49), (64, 81)] <-- note: it and it1 are the same iterator, accessing it1 also advances it and vice versa >>> def median_three(d): d=list(d);return d[:2]+[sorted(x)[1] for x in zip(d, d[1:], d[2:])] ... >>> it = (i*i for i in range(10)) >>> median_three(it) [0, 1, 1, 4, 9, 16, 25, 36, 49, 64] </pre>

koborC on Jan. 5, 2021, 11:54 p.m.
Now I can understand your point! : ) It is new to me that we may not have access to each of iterables' elements once! thank you <3 So our code (I use slice in my code too) should at least call list(...) to transform iterable to sequences to guarantee that it is "subscriptable". . P.S. I learn iterator / iterable is based on <a href="https://en.wikipedia.org/wiki/Iterator_pattern">a design pattern</a>, aim of which is only to traverse collection of data first to last (for once). For this reason, python does not come standard with the ability to access with subscript. This is also interesting, and I share this for people who comes in the future.

List, list and list again

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